An operation of devices for conducting conventional half-duplex communication will be described below.
The device outputs a sending request signal at the time of starting sending and when the sending request signal is outputted, the sending request signal is inputted to a sending permission signal of the self device, so that it is decided that sending can be performed, and data sending is started at once.
However, in the operation described above, two devices run asynchronously, so that sending may be started simultaneously from both of the devices. In this case, a carrier detection signal is inputted to both of the two devices after starting the sending, and at this time, either device suspends the sending by setting of a priority preset.
The device, which suspends the sending, releases the sending request signal and receives data sent from the opponent device and after the completion of the receiving, the device again outputs a sending request signal and resumes the sending.
The device, which wants to start sending, outputs a sending request signal, and its signal is inputted as a receiving-request signal of the opponent device.
When the receiving request signal is inputted, a device of the receiving side outputs a receivable signal since receiving can be performed unless the device of the receiving side is in sending, and its signal is inputted as a sending permission signal of a device of the sending side.
The device of the sending side starts sending of data after the sending permission signal is inputted, so that the two devices can be prevented from starting sending simultaneously by stopping an output of the receivable signal even when the sending request signals are outputted from two ways.
Also, as another conventional example, as shown in JP-A-60-41844 as one example, there is an example of making an error determination when a receivable signal (permission signal) is not inputted within a predetermined time after a sending request signal (channel acquisition request signal) is outputted from a self device.
In the conventional half-duplex communication control method, as small as two control signal lines necessary inside a cable will suffice, but a sending request signal output is used as a sending permission signal input of the self device, so that there was a problem that when sending request signals are simultaneously outputted from the two devices, it may be decided that both the two devices are capable of sending and sending may be started simultaneously from the two devices.
Also, as an operation at the time of sending simultaneously from the two devices, setting of “sending continuation” is previously made in one device and setting of “sending suspension” is made in the other device and thereby at the time of simultaneous sending, sending from the device in which the setting of “sending continuation” is made can be performed, but there was a problem that a setting mistake of the case of setting “sending suspension” or the case of setting “sending continuation” in both the devices cannot be detected.
Also, in the case of another conventional example described above, sending is started after the device of the sending side checks a signal as to whether the opponent device is receivable, so that sending is not started simultaneously from the two devices, but there were problems that since it is necessary to use four signal lines inside a cable as a control signal, the cable thickens and wiring of the cable becomes worse or a price of the cable increases.
Further, there were problems that since a control signal line is monitored by H/W, a complicated circuit for monitoring becomes necessary and a size of a communication device becomes large or a price of the communication device increases.
Therefore, this invention is implemented to solve the problems described above, and an object of the invention is to provide a half-duplex communication control method in which both devices are prevented from starting sending simultaneously and also previous settings of a priority etc. are unnecessary.